In order to screen elements or materials of different sizes, especially those of vegetable origin, such as wood or similar, it is known that the device described in the international patent application WO-A-98/40173, belonging to the same Applicant. This known device comprises a plurality of rolls, all rotating in the same direction, facing each other so that the cusps of each roll are inserted into the corresponding V-shaped grooves of the adjacent rolls, thus defining a zig-zag discharge profile. The rolls are also distanced laterally to define adjustable gaps between them, through which only those pieces which are equal to or smaller than the gaps can pass. Moreover, the connection surfaces between cusps and grooves are worked with protuberances, protrusions, hollows or faceted parts which allow to separate the pieces to be screened better.
Although this known device is very efficient for separating and sub-dividing relatively small pieces, that is to say, of a few millimetres, it is not suitable for separating materials which have large surfaces in proportion to their volume, as is the case with strands or wafers, which although they are relatively thin (about from 0.4 to 1.0 mm), have relatively large other dimensions: a length varying from about 60 mm to about 180 mm and a width varying from about 20 mm to about 80 mm. In fact, they tend to fall prevalently in horizontal layers, incorporating between them both smaller pieces (also called as micro-strands) and also very small pieces, such as slivers (called fine pieces) and also tiny pieces, such as saw dust (called super-fine pieces).
The state of the art also comprises other types of screening devices, such as those with a rotary drum, plane, oscillating or vibrating screens.
Rotary drum screens are not only very bulky, but also they have the problem of low specific efficiency since: only the lower surface is involved in the screening; the holes of the sieves are easily blocked; many long pieces, having however limited width, are erroneously discarded together with the fine pieces because they pass through the holes lying coaxial therewith. Rotary drum screens, moreover, do not allow to modify, simply and quickly, the value of the granulometry to be obtained, since this operation requires the sieves on the periphery of the drum to be completely replaced, and this takes a notoriously long time. The long time during which the strands remain inside the drum and their continuous mixing also generates further fine pieces.
Plane screens, whether oscillating or vibrating, are not able to separate the different layers of strands, which lie one on top of another in a sandwich, and which incorporate the fine materials inside them or retain them above.
DE-C-589557 discloses a screening device wherein a plurality of discs, having circular or elliptical form, are mounted on shafts disposed parallel therebetween and axially staggered, in order to define substantially constant discharge apertures between the opposed discs of two adjacent shafts.
U.S. Pat. No. 6,149,018 discloses an apparatus for sorting recycled material, which comprises a plurality of co-rotating spaced parallel shafts, each of which has a longitudinal series of screen discs. On each shaft the axial distance between the discs is rather high. Moreover each disc is shaped in order to have a constant discharge aperture with respect to the opposed disc during rotation of the shafts.
The present Applicant has devised the method and embodied the apparatus according to the invention to overcome the shortcomings of the state of the art, and to obtain further advantages which will be described hereafter.